The invention relates to an ultrafiltration membrane of linear polyurethane which has a high hydraulic permeability and a high separation capacity. It also comprises a process for making the membrane and a process for using the membranes in order to separate oil from an oil-water emulsion or to bring up the concentration of oil present in an oil-water emulsion.
Ultrafiltration is a separation process in which a dissolved or emulsified material is withdrawn from a solution or emulsion which contains the dissolved or emulsified material which latter has a larger molecular dimension than the solvent. The separation is effected by subjecting the solvent or emulsion during its passage across the surface of a semipermeable membrane to a pressure which forces the solvent to permeate the membrane.
The term "ultrafiltration" is mainly used to describe pressure activated separations in which a material of a molecular weight of 500 or more is separated from the solvent.
Ultrafiltration membranes have been described in a number of patents and other literature of the art. References have also been made in the literature to the use of polyurethanes for making foils and membranes. However, these membranes either had substantial shortcomings or were not particularly suited for ultrafiltration purposes. Thus, Lyman in Academic Sciences 146 (1) 113-8 (1968) describes a synthetic membrane for the hemodialysis. In this publication there has been made mention also of polyurethane which can be used for making the membranes. This type of membrane has, however, a high sensitivity to water, in addition has very poor mechanical properties and is unsuited for ultrafiltration.
In the French Pat. No. 1,355,577 numerous polyurethanes have been described which are supposed to be suited for making microporous foils or films which have an excellent steam permeability. These foils or coatings are, however, exclusively used in making synthetic leather and are entirely useless for ultrafiltration purposes.
In the German accepted application No. 1,794,191 anisotropic microporous polymeric low pressure membranes are described which may also be made for polyurethane. This application, however, does not include any specific reference to the composition or structure of these polyurethanes.
In the German published application No. 2,627,629 membranes made of polyurethanes are described which are intended for the separation of aromatic hydrocarbons from aliphatic hydrocarbons, cycloaliphatic hydrocarbons and other aliphatic compounds. The separation of the aromatic hydrocarbons is carried out in a process which is named "pervaporation". According to the statements in this application all polyurethanes are practically suitable for this type of membrane. Specific references to polyurethanes which might be useful for making ultrafiltration membranes, however, are not found in this application.
Although, therefore, quite a number of ultrafiltration membranes have been known there is still a need for improved membranes which can be used for different purposes and can be manufactured in simple processes.
It is therefore an object of the present invention to provide for an ultrafiltration membrane which can be made in a simple way from linear polyurethanes, and, in addition, have a high hydraulic permeability and a high retention capacity.
Another object of the invention is to provide for ultrafiltration membranes which can be used in a broad pH range, which exhibit their separation action within a broad temperature range and which can furthermore withstand comparatively high pressure without being torn.
An object of the invention is furthermore to provide for ultrafiltration membranes of polyurethane which have a high resistance to clogging, which can be used for many different purposes and permit in particular to separate oil from oil-water mixtures even though the oil may be emulsified in the water which is the case where the oil-contaminated water contains additionally detergents which, as is well known today, make the separation of oil and other contaminations extremely difficult. A further object of the invention is to provide ultrafiltration membranes useful for separating and concentrating water which contains fats or fatty oils as it occurs particularly in the food and feed industry.